Use Ohm's Law to solve for voltage (V), current (I), or resistance (R). Enter any two values and the calculator finds the third. Also shows power dissipation (P = VI).
Ohm's Law is the foundational equation of electrical circuits. Discovered by Georg Ohm in 1827, it relates three quantities: voltage (the electrical pressure pushing charge), current (the flow rate of that charge), and resistance (what limits the flow). The relationship is famously simple — V = I × R — and yet it underlies everything from picking a current-limiting resistor for an LED to sizing residential wiring.
This calculator solves for any one of the three variables given the other two, and also computes power dissipation (P = V × I, or equivalently P = I²R, or P = V²/R). Enter the two values you know and choose which variable you're solving for.
A few notes: Ohm's Law holds for "ohmic" materials, where resistance is constant across the operating range. Most metals at modest currents are ohmic. Many semiconductors (LEDs, diodes, transistors) are not ohmic — their resistance changes with voltage — so Ohm's Law alone doesn't describe them. For DC circuits with resistors, though, it's exact.
An LED requires 20 mA at a 2.1 V forward drop. Power supply is 5 V. Resistor must drop: 5 − 2.1 = 2.9 V Current: 0.020 A R = V / I = 2.9 / 0.020 = 145 Ω → use the standard 150 Ω value P = I²R = 0.020² × 150 = 0.06 W → a standard ¼ W resistor is plenty
A space heater draws 12.5 A at 120 V (typical U.S. wall outlet). Apparent resistance: R = V / I = 120 / 12.5 = 9.6 Ω Power: P = V × I = 120 × 12.5 = 1500 W (the heater's rated power) Standard U.S. 15-amp circuits trip at 15 A, so this heater is close to the limit. Two heaters on the same circuit would trip the breaker — which is exactly why circuits are sized this way.
Use Ohm's Law for any DC circuit with resistive components — designing or analyzing simple electronic circuits, picking resistor values, sizing wiring for a current draw, choosing fuse/breaker ratings, calculating heat dissipation.
For non-ohmic components (LEDs, diodes, transistors), use the device's I-V curve or specific equations (Shockley diode equation, transistor models). For AC analysis with phase relationships, use complex impedance. For high-frequency RF work, transmission-line effects matter.
A practical tip: when in doubt, calculate the power dissipation. Underpowered resistors are the most common point of failure in hobbyist circuits.
Calculate voltage divider output using Vout = Vin x R2/(R1+R2).
Calculate electrical power using P = VI, P = I²R, or P = W/t.
Calculate the current-limiting resistor needed for an LED circuit.
Solve for speed, distance, or time using the formula Speed = Distance / Time.
Calculate force, mass, or acceleration using Newton's Second Law F = ma.
Calculate acceleration from initial velocity, final velocity, and time.
Leave at 0 when solving for voltage
Leave at 0 when solving for current
Leave at 0 when solving for resistance
Voltage
12.00 V
Current
2.0000 A
Power
24.00 W
| Parameter | Value |
|---|---|
| Voltage (V) | 12.0000 V |
| Current (I) | 2.0000 A |
| Current (mA) | 2000.00 mA |
| Resistance (R) | 6.0000 Ω |
| Power (P) | 24.0000 W |
| Power (mW) | 24000.00 mW |
| Formula Used | V = I × R |